Photoelectric exposure meter



' May 27, 1941-- A. LEBER 1 PHOTOELECTRIC EXPOSURE METER Filed Feb'. l0, 1939 Patented May 27, 1941 Application February 1939, Serial No. 255,668

Germany January 10, 1939 4 Claims.

A serious disadvantageof the exposure meters hitherto known consists in that the values obtained by measuring do not correspond to the actual eilect o! the light in the photographic apparatus, because only a mean value of the whole of the light radiated from the object can be caught by these exposure meters. I-t is therefore not possible to obtain directly a predetermined blackening oi' the negative, that is to order the picture in the `approximately rectilineal portion of the gradation curve. This shows, that it is necessary to measure the brightness interval of the portions of the object which are important for the picture so as to be able to ascertain whether the brightness interval is not Atoo large for the emulsion of the light sensitive material used.

The object of theinvention is, to produce an exposure meter which enables the measurement oi a directive light beam with a very small space angle and consequently the determination of the illuminating intensity of individual portions of the object, especially the brightest and darkest portions. Comparison light photometers have already been built for this pulDOse, but the objection to these is the necessity of a source of articial light for comparison and the iluctuating light intensity of the same.

'I'he'inventiou overcomes these objections and renders unnecessary the arrangement of a source of articial light for comparison. This isattainedV according tothe invention in that on the photo-electric exposure meter means are provided, which enable a comparison of the light intensity. of a point to be' measured on the object to be photographed with the light radiation for a region surrounding the exposure meter when in use.

passage I' is filled with a light diiusion means, for example opal glass, vwhereas the light passage 2 allows the unimpeded passage of the light without diiiusion (directive light). Both light passages I and 2 pass through the casing 3 and may be directed towards the observer. For the com``.

' parison of the light passing into the instrument 'through the two light passages I', 2, known light comparing arrangements, such as a Lummer- Brodhun cube, may be used.

According to the form of construction illustrated in Figs. 1 and 2 the light comparing arrangement is formed by a prism 4 which allows Two embodiments of the invention are illus- `trained, by way of example, `in the accompanying drawing. in which- Fig. 1 shows an elevation .of the side of the instrument directed towards the observer whe using the instrument, l

Fig. 2 is a section on line II-II of Fig. 1. y

Fig. 3 shows a modiiied form of construction oi' the instrument in an elevation similar to that of Fig. 1,

Fig. 4 is a section on line IV-IV of Fig. 3.

I and 2 designate the means, which enable a comparison of the illuminating intensity of a point to `be measured on the object to be photographed with the light radiation from a region surrounding the'exposure meter. They are constructed as light passages, of which thelight the light coming from the opal glass I .to reach the eye of the observer after two total reflections by the' surfaces 5 and 6. The directive light entering through the passage 2 reaches the eye of the observer on a straight path through the surface 1. The observer therefore sees the two kinds of light brought together in such a man- 1 er that the directive light fills the central region 'I' and the diiused light lls the surrounding region 6' (Fig. 1). The prism 4 is secured on the casing 3 by means of a strap 8.

According to the form of construction illus-V trated in Figs. 3 and itwo mirrors 9, I 0 are used instead of the prism 4 (Fig. 2), which mirrors bring, for the purpose of comparison, the diiused.

light enteringat I and the directive light passing through 2 into'two adjacent elds, in a similar manner as the prism. To enable the lbrightness of the two adjacent fields to be equalized, darkening means constructed as grey wedges II, I2 are provided. The grey wedges II, I2 arranged on concentric rings are, for this purpose, rotatable relatively to each other and, ac-

cording to the construction illustrated in Figs. 1 and 2, they are connected with the concentric ring I2 and with the circular disc I4. According to the construction illustrated in Figs. 3 and 4 the grey wedge H is connectedwith a ring I5 and the grey wedge I2 with a disc I6. This modified form of construction will be described later.

The disc I4A according to Figs. 1 and 2 carries the y electric measuring instrument I1 for the photo current which instrument participates' in the movements of the disc I4. Il is the hand of the instrumentand is preferably provided with a stop. This hand points-by means of lead lines t9 provided on the disc I4 .to a sector 20 shiftable on the ring I3 and bearing a linear light intensity scale' 2I. .On this sector 20 the stop scale 22 is arranged which registers with'another scale 23 giving the exposure times, the scale 23 being on a sector `'414 also shiftable on the disc i3.

The two sectors 2| and 24 are arranged on the ring I3 in such a manner that `each sector can l be moved independently without displacing the other sector or the ring I3. 0n the other hand. both sectors 20 and 24 participate in any displacement of ring I3. A mark` 23 and a scale 2E, the latter applied directly on the ring I3..

serve for setting to different emulsion light sensitivities.

The current for the measuring instrument I1 is supplied by the photo-cell 21. The photo-cell 21v and the opal glass I lie in the same plane, so that both the photo-cell and also the opal glass embrace the same `space angle for the impinging diil'used light.

In the form of construction illustrated in Figs.

3 and 4 the disc I6 does not carry the measuring instrument for the photo-current, but a seriesor parallel-resistance 2l'. The associated sliding -contact 29 is ilxed on the ring 30. The disc I8 can be rotated by a lever 3| and the ring 3l by a handle 32 (Fig. 3). By the relative displacement of the elements I3 and 34 the photocurrent is strengthened or weakened, so that theA ordinated. The light density scale 4I is, ac-

cording to this form of construction, arranged on a sector 42 which is shiftable on the element 30.

The manipulation of the exposure meter ac- 'cording to Figs. 1 and 2 is as follows:

If it is desired to measure the light density at a point on the object to be photographed, this point is viewed through the light passage l2. If the brightness of the viewed portion of the picture is greater or less than the brightness of the opal glass illuminated by the diffused light, the person using the instrument sees the sections B', 1' (Fig. 1) in diiierent degrees of brightness. The brightnesses of the surfaces 6', 1 are then equalized by correspondingly turning the grey'wedges II, I2 or the ring I3 and the disc I4. The hand I8 of the measuring instrument I1 will be deflected by the diffused light impinging upon the photo-cell 21 during the' taking of the measurement and which is proportional to that impinging upon the opal glass I. By turning the sector 2li the zero point of the scale 2| is set over the corresponding leading line I9 opposite the position of the hand. Previously, by setting the sector 24 with the mark 25 to the corresponding plate sensitivity determined by the material to be used, the instrument has been set in such a manner that the exposure data can be read on the scales 22 and 23. The tables are preferably so gauged that, when measuring the darkest point of the object most important foi the picture. the corresponding blackening occupies the lcwenposition of the gradatlon curve. The brightest pictorially important point of the same object to be photographed can. then be measured, the light contrast of the object beingv then directly readable on the light density scale 2| whose zero point has already been adjusted during the measurement of the darkest pictorially important Point.

The manipulation of the exposure meter illustrated in Figs. 3/ and 4 is as follows:

For example the darkest pictorially important portion of the object to be photographed is viewed through the light passage 2, any inequality of brightness of the comparison iields being equalized `by turning the grey wedges II and vI2 or the ring I3 and disc II. Hereupon If it is desired to carry out a light contrast measuring with this form of construction, the brightest pictorially important point is preferably measured first. Then the zero point of the scale 4I is brought opposite the mark 43. At the subsequent measuring of the darkest pictorially important point the mark 43 indicates on the scale 4I the amount of contrast of the brightness of the object to be photographed. The scales 2I and 4I (Figs. l and 3 respectively, are preferably logarithmic scales.

Difliculties caused during the turning of the comparison fields by different colors of these flelds may be avoided by darkening the two fields -v as much as possible.

casing having a light passage therein constituting a view passage for observing the object points, said casing having a second light-passage wherein said disk is mounted, means for equalizing the intensity of the light passage through said passages, means for measuring the current produced by the photoelectric cell and vcomparison scales each connectedto a respective one of said means for indicating the relative intensity of the light .passage through said passages.

2. The meter of claim 1 whereina third scale' is carried by the casing in parallelism with one of the other scales and indicating theexposure 3. The exposure meter of claim 1 wherein the measuring instrument ls provided with a hand and is provided with operating means for said hand and a resistance for normally maintaining the hand at the zero point of one of said scales.

4. The exposure meter of claim 1 wherein one of the lightpassages is provided with a movable member graduated in resistance Ato the passage of light from one end tothe other.

` A ALoIs LEBER. 

